Because of its high glass transition temperature and rigid molecular chain structure, polyimide has been known as a raw material for separating membrane excellent in heat resistance and chemical resistance, etc., and various separating membranes using polyimides have been studied. For example, U.S. Pat. Nos. 4,378,400 and 4,959,151 disclose aromatic polyimides using biphenyl tetracarboxylic dianhydride, JP-A-5-7749 (the term "JP-A" used herein means Japanese unexamined patent publication), U.S. Pat. Nos. 3,822,202, 3,899,309, 4,532,041, 4,645,824, 4,705,540, 4,717,393, 4,717,394, 4,838,900, 4,897,092, 4932,982, 4,929,405, 4,981,497, 5,042,992, etc. disclose fluorine-containing aromatic polyimides.
Furthermore, polyimide systems using aliphatic or alicyclic tetracarboxylic dianhydrides are disclosed in U.S. Pat. Nos. 4,964,887, 4,988,371, etc.
However, no polyimide satisfies the permeability and separating performance requirements at the same time. On the other hand, U.S. Pat. Nos. 3,822,202 and 4,717,393 disclosed that the fluorine-containing aromatic polyimide composite membranes solve the above problem and satisfy permeability and separating performance at the same time on a uniform membrane level, but they have problems in terms of practical use on a commercial scale because of cost. Thus they cannot be put into a practical use. Moreover, as described in JP-A-62-216622, it is relatively easy to form a composite membrane having a polymer other than polyimide such as polystyrene, or poly-2,6-dimethylphenylene supported on a polyimide supporting membrane. However, in the case where a polyimide thin film is formed on a polyimide supporting membrane, e.g., by coating, the polyimide supporting membrane is dissolved in a solvent for use in coating, etc. Consequently, no composite membrane having a polyimide thin film formed on a polyimide supporting membrane and having a good balance of various performances has yet been known.